1. Field of the Invention
The invention is directed towards a device for high-voltage amplification.
2. Description of the Related Art
High voltage amplifiers (i.e. amplifiers having a voltage swing of greater than about 40-60V) typically require a highly specialized circuit technology to withstand substantial voltage differences. These technologies include double-diffused MOS (DMOS), high voltage MOS (HVMOS) transistors, and high-voltage bipolar transistors referred to herein as high voltage transistors.
A DMOS or HVMOS typically includes at least four terminals: drain, gate, source, and bulk, with the bulk often tied to the source of the transistor. Typical DMOS and HVMOS transistors are engineered to withstand a large voltage between the drain and any of the other terminals. The other terminals are not designed to withstand such large voltages. In particular, the gate of a high voltage MOS transistor typically cannot withstand more than approximately 30V with respect to the source. In addition, high voltage transistors are typically very large in comparison to low voltage devices, since they must allow large distances for high-voltage-induced depletion regions to span.
High voltage amplifiers are useful in applying electrostatic actuation in optical switching arrays, optical beam steering, optical displays, disk-drive head actuators, other actuators, and electron-beam steering for a cathode-ray tube. A well-controlled high voltage amplifier is particularly important for attaining stable and accurate electrostatic actuation, since capacitors used for electrostatic actuation have a force dependent on the square of the voltage across their terminals.
In the aforementioned applications of high voltage amplifiers, it is useful to have an amplifier which is able to provide a well-regulated output voltage that is a multiple of a low-voltage input. The low-voltage input may be derived using low-voltage circuits. Since electronic devices have poor control over parameters such as output resistance and transconductance, a well-controlled, or voltage-stabilized, output voltage requires voltage feedback from the output terminal. However voltage feedback using known devices and methods is lacking. For example, as described above, high voltage transistors are normally large and their gate generally cannot withstand more than about 30V. Hence, switched capacitor techniques are unwieldy with high voltage transistors, and lack the performance that they attain at lower voltages. Resistive feedback, while another option, results in large power dissipation, since power dissipation is proportional to the voltage squared. To reduce power consumption to a level appropriate for highly integrated devices (including optical mirror arrays), resistors may be made large (on the order of tens of Mega Ohms). However, large resistors mandate that closed-loop bandwidth be reduced to maintain stability, since parasitic capacitances will conspire with these large resistances to form low frequency poles. In addition, large-valued resistors are big and diffusion resistors, typically the only resistive elements available that can form a large-valued resistor in a practical amount of space, are poorly controlled over temperature. Furthermore, depletion regions present in diffusion resistors will vary significantly over the operating range causing large nonlinearities. Diffusion resistors also suffer from junction leakage; caution must also be exercised to ensure that reverse-biased junctions do not break down. Hence, it is difficult to manufacture small integrated circuit devices using large resistors.
The present invention, pertains to a unique high voltage amplifier. In contrast to prior art high-voltage amplifiers, the invention described herein uses field transistors to obtain a well characterized and stable voltage transfer characteristic, with a minimal amount of power consumption, in a small area. In addition to low power consumption and small footprint, the inclusion of field transistors for voltage feedback typically will require no process modification, since parasitic field transistors are created in standard CMOS technologies.
The invention finds particular applicability in driving optical mirror arrays and other applications where a stable, high voltage amplifier controlled by a small input voltage is required.
The invention, roughly described, comprises a circuit which, in one embodiment, includes at least one low voltage input, at least one high voltage output, and a first field transistor having a source, a drain and a control region, wherein said control region is connected to said high voltage output.
In a further embodiment, the invention comprises a high-voltage amplifier. In this embodiment, the invention includes an input terminal, a high-voltage output terminal, a first field transistor having a gate a source and a drain, a second field transistor having a gate a source and a drain, an electrical connection between said high-voltage output terminal and said first field transistor gate, and an electrical connection between said input terminal and said second field transistor gate. Various embodiments of the field transistor are described.
In a further embodiment, the field transistors are provided at different sizes, wherein the size ratio of the transistors is proportional to the gain of the amplifier.
In yet another embodiment, the high voltage amplifier includes a transimpedence stage, comprising an output voltage responsive to an input current which may be single-ended or differential.
In a still further embodiment, a current differencing circuit is provided, and is coupled to the drain of said first field transistor. In this embodiment, the current differencing circuit is coupled to a trans-impedance stage providing a voltage output to an output terminal. The invention may further include a difference current amplifier having at least one transistor mirroring and amplifying the difference current in said amplifier.
In another embodiment, the invention comprises an integrated circuit including a high voltage amplifier circuit. The high voltage amplifier includes a high voltage core having a first terminal and a second terminal, a common mode feedback circuit and a differential mode feedback circuit. The common mode feedback circuit includes a first field transistor and a second field transistor, each transistor having a control gate, the control gate of the first field transistor coupled to said first terminal and said control gate of said second field transistor coupled to said second terminal, respectively. The differential mode feedback circuit includes a differential input and a third field transistor and a fourth field transistor, each transistor having a gate, the gate of said third field transistor coupled to said first terminal and said gate of said fourth transistor coupled to said second terminal.
In a still further embodiment, the invention comprises an optical mirror array, including at least one MEMS mirror and a high voltage amplifier on an integrated circuit.
These and other objects and advantages of the present invention will appear more clearly from the following description in which the preferred embodiment of the invention has been set forth in conjunction with the drawings.